As one skilled in gas turbine engine technology will appreciate, it is abundantly important to maintain the engine case, particularly to those cases of a module type of engine construction that support the engine's shaft or rotating components, precisely concentric throughout the engine's operation and over extended periods of time. While this invention was conceived for a particular purpose of correcting a problem where the engine case grew out of concentricity on existing engines, it is intended to be incorporated into originally designed engine cases as well.
To more fully appreciate the problem confronting the inventor at the time the invention was conceived, reference will be made to FIG. 1 that exemplifies the prior art engine cases. Typically, the adjacent engine cases are tied together by nut and bolt assemblies (not shown) secured through the cooperating apertures 14 and 16 formed in the mating flanges 10 and 12 of case 18 and 20, respectively. The axial extending lip 22 is snap fitted on the inner diameter 24 of case 20. Under certain circumstances, particularly when both engine cases are made from the same material that exhibit identical coefficients of thermal expansion, the cases grow and shrink uniformly without causing distortions. However, in order to reduce weight of the engine and improve engine operating performances, current engines employ cases made from different materials and hence exhibit different coefficients of thermal expansion. For example, one case may be made from stainless steel and the other case may be made from a magnesium alloy.
In the present design the materials are different and have different coefficients of thermal expansion and have experienced yielding in case 18 causing the fit to loosen by as much as 0.030 inch. This yielding is attributed to several causes, namely, temperature increase, differences in thermal expansion and a relatively low yield strength of case 18. Since case 18 carries the support mechanism for the bearings that support the engine shaft, it is readily understandable how this yielding is unacceptable and has resulted in bearing failure due to the unacceptable misalignment between case 18 and case 20.
Prior to the conception of this invention, certain solutions were unsuccessfully attempted to fix this problem. Amongst those tried were 1) "softening" the case 20 in the area where the two cases are snap fitted; 2) thickening case 18; changing the material of case 18; and 3) machining axial slots in case 18 adjacent the snap fit.
"Softening" of case 20 and thickening case 18 failed to reduce stresses to an acceptable level. Changing the material of case 18 with a higher yield strength resulted in an undesirable weight increase and prohibited the use of existing hardware when considering the redesign of cases for existing engines. The axial slot configuration which was intended to eliminate hoop stress carrying capabilities was unsatisfactory because it was unreliable due to the low strength of case 18 and the fingers formed as a result of the axial slots deformed excessively.
I have found that I can obviate the problems enumerated in the above paragraphs by providing a thermal load relief ring judiciously fitted into the case at discrete positions relative to the snap fit areas which not only result in the successful repair of existing and non-conforming parts, the redesign of highly-stressed problem areas, but it also evidenced significant weight reductions on new design of cases for gas turbine engines, other types of engines or rotating machinery where maintaining concentricity is deemed important or critical.